Auto transformer help please

[Ian ParkinParticipant @ianparkin39383]

I have the job for a friend to install a secondhand Wolf (us) cooker hood a great big thing 48 inches wide and 60 kg..almost all stainless it has 3 white lights tungsten and 2 heat lights and a big fan motor..

theres a transformer in the hood a auto transformer

the person who uninstalled the hood has cut wires and also disconnected the wires from the transformer to its terminal block ( unnesassarily)

How do i determine which wires are which?

I assume the earth wire is correct?

the white wire that i have connected to 0v (input) and the brown wire that i have put into 240v (input) has continuity

the white wire that i connected to 0v (output) and the blue wire that i put into 115 (output) has continuity

no continuity between any other wires

if i connect 240v to the input it blows the fuse

how should i connect this?

[Michael GilliganParticipant @michaelgilligan61133]

It should be easy, Ian

[ but as I know all-too-well that is rarely true ]

A classic Auto-Transformer has one winding, which is tapped … giving it the same geometry as a potentiometer.

I would therefore expect the two 0v connections to be common.

MichaelG.

.

Edit: __ Just found this, which gives updated correspondence details:

https://www.repancobartlett.co.uk/

[noel shelleyParticipant @noelshelley55608]

Should be easy !First 0v to neutral, 230v to live, obviously earth.  wire your load to second 0V and 115v. AH I SEE /UNDERSTAND ! YES ! Right, try blue to first 0v brown to 230v, Load between the 2 white wires at 115v. To test, see if you have a smallish resistance between blue and brown then the 2 white wires ! connect blue 0v and brown 230v. Fuse holds ? if yes then test for 115v between the 2 white if yes the connect the load. Bear in mind that you are working with lethal voltages so be very carful. Or try contacting the makers . If it is an auto trans former then both 0V will be common and you will have increasing resistance between 0v, 115v, and 230v.  Noel.

[Ian ParkinParticipant @ianparkin39383]

What resistance should one expect on a primary and secondary winding?

 

[Michael GilliganParticipant @michaelgilligan61133]

No idea of actual values, Ian … but to a first approximation the 0-230v reading should be twice the 0-115v reading

MichaelG.

[Ian ParkinParticipant @ianparkin39383]

Right connected 240v on the blue and brown

and theres 240v on the 2 whites with no load

but no resistance between the blue and brown and the pair of whites

do i need to load the output?

[Ian ParkinParticipant @ianparkin39383]

Michael theres 1 ohm and 0.9 ohm between the brown and a white and the blue and a white

[Ian ParkinParticipant @ianparkin39383]

If i load the output which is at 240v with a 240v 150watt light bulb the voltage drops to 45v

[Michael GilliganParticipant @michaelgilligan61133]

Doesn’t sound right to me, Ian … but measuring coils with a DC meter is always tricky.

The new version of the transformer appears to carry the same part number as yours, so I would recommend asking Repanco Bartlett

MichaelG.

.

Crossed with your most recent post

[Dave HalfordParticipant @davehalford22513]

As said above an autotransformer only has one winding with several taps off it. the input goes across the full winding, the output goes between the neutral (0v) and one of the taps suppling the required voltage.

If the sticker is correct the colours going to the transformer do not reflect the mains cable colour code.

Therefore avoiding confusing colours – mains neutral to 0V input mains live to 230V input and a light bulb load across the output should give 115V

[Ian ParkinParticipant @ianparkin39383]

Took the end covers off

is this any help blue and brown on the outside of the windings whites on the inside

[Robert Atkinson 2Participant @robertatkinson2]

mains neutral to input white
Mains live to input brown

Hood neutral (should be white) to output white
Hood live (black) to output blue.

Earth to earth

Bigger issue: fan wil be rated for 60Hz so may overheat on UK 50Hz supply.

Robert.

[Michael Gilligan]

On Michael Gilligan Said:

It should be easy, Ian

[ but as I know all-too-well that is rarely true ]

A classic Auto-Transformer has one winding, which is tapped … giving it the same geometry as a potentiometer.

I would therefore expect the two 0v connections to be common.

MichaelG.

.

Edit: __ Just found this, which gives updated correspondence details:

https://www.repancobartlett.co.uk/

Ian’s transformer is actually shown on that page, lower right.

I know how I would try wiring it, but I also have the advantage of owning a variac to experiment; a quick phone call should suffice.

[Michael GilliganParticipant @michaelgilligan61133]

Bill

… Great minds ?

https://www.model-engineer.co.uk/forums/topic/auto-transformer-help-please/#post-712773

MichaelG.

.

The new version of the transformer appears to carry the same part number as yours, so I would recommend asking Repanco Bartlett

[Ian ParkinParticipant @ianparkin39383]

Robert and others

remember that i have wired the wires into the terminal block so they may not be right

the previous uninstaller had disconnected the transformer….why I don’t know

theres 4 wires coming out of the transformer and 4 terminals   (Plus an earth to body)

so i would guess that theres a 240v in to the ones marked 0v and 240v input

and 110v to the ones marked 0v and 110v output

but should the 2 0v’s be common?

[Ian ParkinParticipant @ianparkin39383]

Robert and others

remember that i have wired the wires into the terminal block so they may not be right

the previous uninstaller had disconnected the transformer….why I don’t know

theres 4 wires coming out of the transformer and 4 terminals   (Plus an earth to body)

so i would guess that theres a 240v in to the ones marked 0v and 240v input

and 110v to the ones marked 0v and 110v output

but should the 2 0v’s be common?

[duncan webster 1Participant @duncanwebster1]

I’d get hold of a low voltage transformer to experiment with. If you connect 240v where you should have 115 you’ll have nearly 500 v on the other terminal, which is very good at killing you. If you tell us your approx location someone might be able to lend you one

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Ian, you should have continuity between all four connections between 0-V and 230-V with some resistance between each, but as has been said, the two 0-V’s might common so will have no resistance between them, if not, there is probably an internal fault, and that maybe why the wires were cut. There should be no continuity between any of the four connections and the Earth connection at all.

Wiki link https://en.wikipedia.org/wiki/Autotransformer

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

Ian

Sorry to labour this, but Nick has provided that useful Wikipedia link, which will conveniently take you here:

https://en.wikipedia.org/wiki/Autotransformer#/media/File:Tapped_autotransformer.svg

That illustration, although it depicts multiple tappings, should make it clear how the thing needs to be connected. … Yes, the two 0v connections should be common … and a tapping half-way up towards the 230v input on the Primary is your 115v output.

 

Always assuming that the device is not faulty.

MichaelG.

.

Edited for better verbal clarity … I hope.

 

 

 

[Ian ParkinParticipant @ianparkin39383]

Michael

 

i think i now have connected it in every combination of supply and out puts

theres 4 wires 2 whites and one blue and one brown..

one white has continuity with brown

the other has continuity with blue

the brown and blue don’t have continuity with each other

the whites don’t have continuity with each other

continuity is 1 ohm or so..

 

i thought i knew what a auto transformer was

i have a few and a variac and can see how you apply 240 across the whole winding and then tap off from the 0v to the wiper..

surely this auto should be the same..

All the windings look good inside.

I’ve rung the manufacturer but no one there at the moment to help me

[Ian Parkin]

On Ian Parkin Said:

…

 

surely this auto should be the same..

…

That would be my understanding, Ian

Sorry, I’m  lost for any other suggestions.

MichaelG.

.

Edit: __ [removed after responding to Simon’s post]

[noel shelleyParticipant @noelshelley55608]

Where abouts are you Ian ? The question is first what do blue, brown and the 2 whites do ? bearing in mind that they were disconnected  from the terminal block by someone so there current positions are your guess. Putting mains on blue and brown didn’t blow the fuse and gave an output ? Blue and ONE of white wire may well show 0 resistance, Blue and THIS wire are the two 0v ! The other white will be 115v, with brown as 230v. As has been said measuring low resistances with an ordinary multi meter is not easy or good but should give a guide. The autotransformer is rated for 50/60Hz and the unit is secondhand so the lamps will be happy on our 50Hz and the fan has either worked for years on 50Hz or had it – you’ll soon know !  Good luck Noel.

[Frances IoMParticipant @francesiom58905]

it is a 115v 1:1 transformer that can be wired as a auto transformer (just get the phasing of the windings correct!) – Robert Atkinson has given the connections – suspect someone has rewired the terminals

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Ian, it is possible that there is a thermal fuse buried inside the windings that joint the two 0-V’s together, and if the transformer has been subject to an overload due to a fault in the hood, then the fuse would probably blown. If it has had a thermal fuse that’s blown, it probably won’t be replaceable.

You could try connecting a fuse between the two 0-V output terminals, in a suitable insulated housing, and then connect a 230 V light bulb between the 0-V input and the 0-V output, with the mains on 0-V & 230 V inputs, which should then light up the bulb at about half its normal brightness if an internal fuse has blown.

Regards Nick.

[Simon Williams 3Participant @simonwilliams3]

I fear the suggestions on how this should be resolved have not taken enough notice  of the OP’s comment that we don’t know the four wires coming from the transformer innards are connected to the correct labels.  We should ignore the text of the label and start from first principles.

Ignore the green and yellow wires for the time being.  All four conductors – two whites, a brown and a blue – should have DC conductivity between them.  If not the unit is either faulty or it isn’t an auto transformer.

One would expect the blue and the brown wires will have the greatest DC resistance between them.  If so these are the tappings for 0 volts and 230 volts respectively, though this is an assumption.  Go with it for the time being.

If this is correct one of the white wires is the output 0 volts, and thus is directly connected to the blue neutral.  Check the DC resistances and see if one of the white wires has a resistance to the blue wire of as close to zero ohms as the OP can measure.  If so we’re talking auto transformer, if not we’re in a different world entirely.

By a process of elimination the remaining white wire is the nominally centre tap of the auto-transformer winding, and should measure approx equal resistance between it and the blue neutral and between it and the brown live input.

Now confirm there is a low resistance connection (<1 ohm) between the green/yellow wires and the frame of the transformer, also there is a high resistance (Megohms) between any of the power conductors and the frame.

If all this works out, now shuffle the wires to their respectively labelled connector blocks.

The OP has reported having successfully applied 230 volts to the unit without blowing fuses though there does appear to be an anomaly with respect to the output voltage and its behaviour under load.  This may be indicative of a fault within the transformer.

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